1. Field of the Invention
The present invention relates to a telecommunication system, and more particularly to an asymmetric digital subscriber line (ADSL) system using a digital-subscriber line coexisting with a communication line for a time-compression multiplexing integrated services digital network (TCM-ISDN) line system.
2. Discussion of Related Art
In recent years, communication methods that are capable of facilitating high-speed data communications and minimal installation costs and user fees have become imperative to meet requirements for high-speed communications via Internet between users of personal computers. As a result, a Digital Subscriber Line and its variations (xDSL) system has been proposed. The xDSL system enables digital data communications by using existing general copper phone lines installed in homes and offices.
The “xDSL” system, which generally refers to all kinds of communication methods using phone lines, includes a High data-rate DSL (HDSL) system taking the place of existing T1 lines, a Symmetric DSL (SDSL) system taking the place of T1 or E1 lines by using a single twisted-pair copper line, an Asymmetric DSL (ADSL) system capable of transmitting high-capacity data by a public switched telephone network (PSTN).
In an ADSL system, the word ‘asymmetric’ is named because downstream data transmitted from a central office (CO) to remote terminals (RT) has wider bandwidth and more capable of data transmission than upstream data transmitted from the remote terminals to the central office. The ADSL system uses current phone lines or telephones as it is and enables high-speed data communications. Also, in the ADSL system, it is capable of data communications and a Plain Old Telephone Service (POTS), simultaneously. The ADSL system provides a transmitting speed of up to 8 Mbps in a downstream direction and that of up to 640 Mbps in an upstream direction.
The transmission of data in an xDSL system needs wider bandwidth and thus creates higher cross-talk interference between copper pairs in the same cable-binder group. The level of the crosstalk interference varies depending on different cable structures and materials. In particular, some countries such as Japan and Korea use telephone cables with a paper-based “pulp” insulator rather than plastic-insulated cables (PIC) used in the United States. These pulp cables produce much more cross-talk interference than the PIC cables. Thus, it is more difficult to provide wide-band DSL services in those countries which use the pulp cables because their existing telephone cables are prone to crosstalk interference.
FIG. 1 is a diagram of conventional ISDN line telecommunication system which has interference.
A central office 10 contains several Integrated Services Digital Network (ISDN) line cards 11 that connect a telephone network backbone to. local lines 20 that are strung to equipments of users. Remote ISDN terminal adapters or modems 30 are located at different remote customer sites within a few kilometers of the central office 10.
Local lines from ISDN line cards 11 to remote ISDN modems 12 are routed through one or more cable bundles 12. These telephone-cable bundles 12 may contain dozens or more separate telephone lines or copper pairs. Standard voice services, ISDN services, and newer DSL services share the same cable bundle. Since lines run close to other lines in cable bundles 12, mutual inductances can create cross-talk interference or noise between lines 20. For voice services such as Plain Old Telephone Service (POTS), frequencies are so low that interference is negligible. However, interference cannot be neglected in ISDN digital services using a higher bandwidth of around 80 to 320 kHz. New xDSL services also use higher bandwidths. For example, bandwidths of an ADSL system are typically above 1 MHz and have significant cross-talk interference. To prevent this cross-talk interference in countries such as the U.S., where better-insulated PIC cables are used, a full-duplex data transmission system having echo cancellation has been employed.
To eliminate the cross-talk interference in countries such as Japan, where the pulp cables are installed, a time-compression multiplexing (TCM) ISDN line system is used rather than the full-duplex data transmission system having echo-cancellation. The Operation of TCM-ISDN line system is based on a TCM timing reference (TTR) signal of 400 Hz. In such a system, the ISDN line cards at the central office transmit data when the TTR signal is at an active high. The ISDN line cards all receive data from ISDN modems during a different time period. Thus, near-end cross-talk (NEXT) interference is eliminated because none of the other ISDN cards at the central office are receiving data during the transmission time-period. Although far-end crosstalk (FEXT) interference still exists, it is usually much weaker than NEXT interference.
FIG. 2 is a timing diagram for a TCM-ISDN line system. During window 22, data is output from a central office to a remote ISDN modem installed in customer premises. This data arrives at the remote modem after a predetermined delay, during reception window 24. An ISDN equipment of the customer premises includes a burst clock detector (not shown) to determine the timing of a receive downstream burst and to generate the timing for a transmit upstream burst thereof. During window 26, upstream data is transmitted from the remote modem to the central office, which arrives at the central office after a predetermined delay, during window 28. At any particular time, only one end of the TCM-ISDN line system is transmitting data, while the other end is receiving data. An echo cancellator is not needed because a transmitted signal does not have any echo that has to be removed.
As an ADSL system as well as the existing public switched telephone network (PSTN) enable super-speed communications, data communications under other circumstances such as established ISDN and TCM-ISDN has been proposed so that an ADSL Annex B system and an ADSL Annex C system were developed. The ADSL Annex B system is a variation of the ADSL system in which frequency bandwidth of upstream data is not contained in that of ISDN to use the ADSL system under ISDN. The ADSL Annex C system is designed for using the ADSL system under TCM-ISDN of half-duplex data transmission system used in countries such as Japan.
ADSL services use a full-duplex data transmission system. Therefore, a receiver at either side receives data all the time. If such an ADSL modem and the TCM ISDN are installed in the same cable bundle, the strong near-end cross-talk (NEXT) interference due to TCM ISDN modems will severely affect the reception of the ADSL signal during the transmission of data.
FIG. 3 is a diagram of interference at a central office when several ISDN lines transmit data at the same time. During transmit window 22 of FIG. 2, a burst of data is sent from the central office to remote sites. Near-end crosstalk (NEXT) interference of the ADSL modem at the central office is particularly strong during transmit window 22, when the ISDN devices at the central office are all transmitting data. During receive time window 28, these ISDN devices at the central office are not transmitting data. Far-end-crosstalk (FEXT) interference is weaker than NEXT interference because it is attenuated by the length of the telephone line. However, in the customer premises ISDN modem, near-end crosstalk (Next) interference during the transmission of data is stronger than FEXT interference during the reception of data.
Accordingly, the ADSL modem installed with TCM-ISDN in the same cable bundle has to be designed such that ADSL signals are transceived in consideration of FEXT interference and NEXT interference. Dual bit-map (DBM) and FEXT bit-map (BBM) minimize NEXT interference. DBM uses different bit-map according to FEXT/NEXT interference period, and BBM which uses singular bit map transmits data only at a period of FEXT interference which is weaker than NEXT interference. When ADSL services are provided in a situation of TCM-ISDN, Next interference is stronger than FEXT interference throughout the whole bandwidth. In this situation, precise network time synchronization can be performed between ADSL and TCM-ISDN services, and data transmission may be maximized when data is transmitted in a DBM type.
As described above, in TCM-ISDN, data is transmitted from a central office to a remote terminal when a TTR signal is at a high level. Inversely, data is transmitted from the remote terminal to the central office when the TTR signal is at a low level. However, since ADSL modem cannot directly use the TTR signal of the TCM-ISDN modem, the TTR signal is transmitted as the downstream data with a certain frequency during the initialization. For example, the central office according to ADSL Annex C transmits pilot tone of 276 kHz and TTR indication tone of 207 kHz at the same time. The remote terminal performs clock synchronization using the pilot tone. According to a hyperframe synchronized with the TTR signal, phase information of +45° during FEXT interference period and phase information of −45° during NEXT interference period are included in the TTR indication tone by the central office. Accordingly, the remote terminal analyzes a received TTR indication signal of 207 kHz and determines the phase information on FEXT interference and NEXT interference periods to constitute a hyperframe of a received frame.
FIG. 4 is a diagram illustrating hyperframe symbols of ADSL downstream signals, and 345 symbols constitute one hyperframe. One hyperframe includes 345 frames and 1 symbol represents a symbol of a frame. Here, a period of a hyperframe is 85 ms, which is a multiple (i.e. 34 times) of a period (2.5 ms) of a TTR signal. Although FIG. 4 illustrates an example of a hyperframe including a cyclic-prefix, the foregoing description may be also applied to a hyperframe including no cyclic-prefix. But, at this time, a period of one hyperframe (345 symbols) is 80 ms.
FIG. 5 is a table illustrating one hyperframe of an ADSL signal during 34 periods of a TTR signal. A slant-lined region is a symbol representing FEXT interference period, and the other region is a symbol representing NEXT interference period. Here, a symbol is determined to be FEXT interference or NEXT interference on the basis of vertical dotted lines.
FIG. 6 is a timing diagram exemplarily illustrating that a remote terminal recognizes a symbol included in a TTR indication signal transmitted from a central office. The ADSL central office according to ADSL Annex C is synchronized with the TTR signal of TCM-ISDN during the communication initialization and generates a hyperframe. The ADSL central office stores symbols of the hyperframe in a signal of 207 kHz and transmits the symbols as a TTR indication signal TTR_I. When the TTR signal is at a high level (i.e. during NEXT interference period), a phase of the TTR indication signal TTR_I leads 45° ahead of that of an original signal. On the other hand, when the TTR signal is at a low level (i.e. during FEXT interference period), a phase of the TTR indication signal TTR_I lags 45° behind that of the original signal.
The remote terminal interprets the TTR indication signal TTR_I to determine symbols of the hyperframe. If the remote terminal starts to receive from A point the signal transmitted from the central office, the remote terminal may detect phases of frames 2000-2004 and precisely determine a symbol of each of the frames 2000-2004 whether it is FEXT interference or NEXT interference. As a result, data inputted after the TTR indication signal can be precisely decoded to data of FEXT interference or NEXT interference period.
However, in the case that the remote terminal of customer premises (i.e. ADSL modem) starts to receive from B point the TTR indication signal (TX) transmitted from the central office, a phase of the frame 1003 is different from those of other frames 1000-1002, and 1004. Thus, a symbol of the frame 1003 cannot be determined as to whether it is FEXT interference or NEXT interference. It is. impossible to precisely determine whether data inputted after the TTR indication signal TTR_I is data of FEXT interference period or data of NEXT interference period. As a result, data cannot be normally decoded.
A need therefore exists for a full-duplex telecommunication system capable of precisely detecting a start position of a data signal inputted after initialization, when the telecommunication system uses a communication line positioned in vicinity of a communication line of a telecommunication system adopting half-duplex data transmission system according to a reference signal.
There is a further need for an ADSL telecommunication system capable of precisely detecting a start position of a data signal inputted after a TTR indication signal, when signals are transceived via a digital subscriber line (DSL) installed with a communication line of TCM-ISDN system in the same cable bundle.